Technical Field
The present invention relates to heat-expandable microspheres, the process for producing the same and application thereof.
Background Art
Heat-expandable microspheres composed of a thermoplastic resin shell and a blowing agent encapsulated therein are generally called heat-expandable microcapsules. The monomers constituting the thermoplastic resin usually include vinylidene chloride, (meth)acrylonitrile monomers, and (meth)acrylate monomers. The blowing agent mainly used includes hydrocarbons such as isobutane and isopentane (refer to PTL 1).
The vapor pressure of the blowing agent, i.e., a hydrocarbon, may change due to the heat generated at the polymerization step in the production process of the heat-expandable microspheres. The change in the vapor pressure may cause expansion and/or compaction of the heat-expandable microspheres being polymerized, and thus may make the shell of microspheres nonspherical, in other words, concave or distorted.
Such concave heat-expandable microspheres have a lower packing efficiency than that of spherical heat-expandable microspheres, and cannot be uniformly mixed with a resin or may flow out of the mixture when the microspheres are mixed with the resin to be prepared into a resin composition, such as a masterbatch. Thus, a sufficient amount of heat-expandable microspheres cannot be mixed with the resin resulting in considerably poor workability of the materials in the preparation of a resin composition.
In addition, heat-expandable microspheres having concave or distorted shapes do not expand sufficiently because the pressure by the blowing agent is not applied uniformly to the shells of the microspheres. Such concave or distorted heat-expandable microspheres in a resin composition are subjected to nonuniform shear stress when the resin composition is processed in extrusion molding, injection molding, calendering or pressure forming where the composition is subjected to high pressure. The nonuniform shear stress imparted to the heat-expandable microspheres may break the heat-expandable microspheres such that the microspheres are poorly expandable in the molding of the resin composition.
Thus there is a strong demand for heat-expandable microspheres having a improved sphericity for solving the problems mentioned above.